A copper thiolate centre for electron transfer: Mononuclear vs. dinuclear complexes

Article abstract of DOI:10.1039/c2dt12355j

We have structurally and spectroscopically investigated a rare example of a mononuclear aliphatic dithiolate Cu^II complex characterized by a reversible Cu^II/Cu^I redox couple. By DFT, we have shown that this system has a lower reorganization energy than its previously described bis(μ-thiolato) dicopper parent complex, which reversibly cycles between the Cu^1.5Cu^1.5 and Cu^ICu^I redox states. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt12355j

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COMMUNICATION
A copper thiolate centre for electron transfer: mononuclear vs. dinuclear
complexes†
Marcello Gennari,a Jacques Pécaut,b Marie-Noëlle Collomba and Carole Duboc*a
Received 6th December 2011, Accepted 6th January 2012
DOI: 10.1039/c2dt12355j
L′2− ligand, leading to the formation of the dicopper [Cu2L′] (1)
complex (Scheme 1). 1 can reversibly cycle between the
We have structurally and spectroscopically investigated a
rare example of a mononuclear aliphatic dithiolate CuII
Cu1.5Cu1.5 and CuICuI states; both formal states, 1+ and 1, have
been crystallographically characterized.7 Here, we have isolated
and characterized a mononuclear dithiolate Cu complex from
L2−, [CuIIL] (2) (Scheme 1). The electrochemical investigation
of 2 has demonstrated the reversibility of the [CuIIL]/[CuIL]− (2/
2−) redox couple, allowing us to electrogenerate an unprece-
dented mononuclear aliphatic thiolate CuI complex, which has
crystallized in the presence of K+ as the hetero-tetranuclear
[Cu2K2L2(THF)2] complex (3). Finally, we have exploited the
unique opportunity to compare the reorganization energy
between the reduced and oxidized forms of these two parent
copper systems, dithiolate monocopper (2/2−) vs. dithiolate
dicopper (1+/1) by calculating their self-exchange inner-sphere
reorganization energies.
complex characterized by a reversible CuII/CuI redox couple.
By DFT, we have shown that this system has a lower reorgan-
ization energy than its previously described bis(μ-thiolato)
dicopper parent complex, which reversibly cycles between
the Cu1.5Cu1.5 and CuICuI redox states.
Investigation of aliphatic thiolate transition metal complexes has
grown into an intense area of research, in a large part because of
their fundamental and necessary role in numerous biological pro-
cesses.1,2 In particular, copper thiolate complexes are used by
nature as efficient electron transfer agents. They are found as
mononuclear type 1 sites, cycling between the CuII and CuI oxi-
dation states, in blue copper proteins (plastocyanin, azurin).3 On
the other hand, a bis(μ-thiolato) dicopper complex, named CuA,
cycles between the Cu1.5Cu1.5 and CuICuI states, in cytochrome
c oxidase and nitrous oxidase reductase.4 The presence of at
least one copper–thiolate bond in these sites leads to peculiar
spectroscopic properties related to their specific electronic struc-
ture, which has been investigated in great detail to tentatively
explain the efficiency of these copper complexes as electron
transfer agents.5 It has been shown that the dinuclear CuA site is
much more efficient for transferring electrons than the mono-
nuclear type 1 sites.6 This has been explained by a better deloca-
lization of the unpaired electron over the {Cu2S2}+ core in CuA,
especially with the presence of a Cu–Cu bond, and by a lower
reorganization energy for CuA with respect to the mononuclear
CuII complex.
The L2− ligand reacts in THF with CuCl2 leading to the
immediate formation of a dark green precipitate corresponding to
[CuL] (2) (Scheme 1). The X-ray structure (Fig. 1) unambigu-
ously establishes the monomeric character of 2, which consists
of a distorted square planar CuIIS2N2 unit, isostructural to that of
the previously described [NiIIL]8 and [ZnIIL]9 complexes, with a
pseudotetrahedral twist angle (Cu–S–S/Cu–N–N) of 35°. The
structural features of 2 compare well with those reported for
related cis-CuIIS2N2 unit structures, with a larger deviation from
planarity for the six-membered copper-aza-thiacyclohexane ring
with respect to the more common five-membered copper-aza-
thiacyclopentane ring complexes,10–12 combined in the case of 2
with the presence of the rigid bipyridine unit. The Cu–S bonds
of 2 are longer than in type 1 sites (Cu–Scys: 2.1 Å), due to the
difference in both geometry (square planar vs. tetrahedral), and
the sulfur environment in the coordination sphere (bisthiolate vs.
thiolate–thioether).
Recently, we have isolated and investigated a CuA model
complex which mimics most of the structural features and the
redox properties of CuA.7 Indeed, in the presence of CuCl,
the ligand L2− reacts with CH2Cl2 to produce the dinucleating
The X-band EPR spectrum of 2 recorded in DMF (Fig. 2) dis-
plays a typical quasi-axial S = 1/2 signal demonstrating that the
mononuclear structure is retained in solution. The g values are
consistent with a dx2−y2 ground state. The EPR properties of 2
are similar to those of the reported CuIIS2N2 complexes.10−13
Note that the presence of nitrogen superhyperfine splitting,
absent for the blue copper proteins, in addition to the relatively
small g value in 2 (g near 2.20 for type 1 copper)14 suggests a
strong nitrogen character in the single occupied molecular
orbital.
The electrochemical behavior of complex 2 has been investi-
gated in DMF. The cyclic voltammogram shown in Fig. 3
aUniversité Joseph Fourier Grenoble 1/CNRS, Département de Chimie
Moléculaire, UMR-5250, Institut de Chimie Moléculaire de Grenoble
FR- CNRS-2607, BP-53, 38041 Grenoble Cedex 9, France. E-mail:
carole.duboc@ujf-grenoble.fr
bLaboratoire de Reconnaissance Ionique et Chimie de Coordination,
Service de Chimie Inorganique et Biologique, UMR E-3 CEA/UJF,
INAC, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
†Electronic supplementary information (ESI) available: Additional
experimental (synthesis, crystallographic and spectroscopic data) and
computational details. Crystal and refinement data for 2 and 3 in CIF
format. CCDC reference numbers 854562 and 854563. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2dt12355j
k
k
3130 | Dalton Trans., 2012, 41, 3130–3133
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Scheme 1
Fig. 1 Molecular drawing of [CuL] (2). The thermal ellipsoids are
drawn at 50% probability level. All hydrogen atoms are omitted for
clarity. Selected bond distances (Å): Cu(1)–S(1) = 2.227(1), Cu(1)–S(2)
= 2.227(1), Cu(1)–N(1) = 2.014(4), Cu(1)–N(2) = 2.016(4).
Fig. 2 Experimental (red line) and simulated (blue line) X-band EPR
spectra of 2 in DMF at 100 K. Parameters for simulation: gk(Cu) = 2.098,
g⊥(Cu) = 2.013, A
= 160 × 10−4 cm−1, A⊥(Cu) = 22 × 10−4 cm−1
,
k(Cu)
A
k(N) = 5 × 10−4 cm−1, A⊥(N) = 10 × 10−4 cm−1
.
displays a single one-electron reversible reduction process
located at E1/2 = −1.2 V vs. Fc/Fc+ (ΔEp = 60 mV), and a poorly
reversible oxidation process at Epa = −0.12 V. Bulk electrolysis
at 0.0 V shows that the oxidized species is not stable. Therefore,
we cannot conclude whether the oxidation is a metal- or ligand-
based process. The one-electron reduced form of 2, 2−, quantitat-
ively generated by exhaustive electrolysis at E = −1.3 V
(Fig. 3b) is EPR silent, consistent with the formation of a CuI
complex. The reduction process is associated with significant
changes in UV-visible absorption (ESI†) with the drastic
decrease in intensity of two intense features of 2 at 575 nm (ε =
2600 M−1 cm−1) and 382 nm (ε = 7900 M−1 cm−1), confirming
the metal-based reduction process. The complete reversibility of
the 2/2− redox couple is indicative of the retention of mononu-
clearity for 2−, and of its notable stability.
NHE15 converted to −0.33 V vs. Fc+/Fc16) and the dinuclear
centre of CuA (+0.24 V vs. NHE17,18 converted to −0.44 V vs.
Fc+/Fc) display the highest redox potentials. The low redox
potential of 2/2− can be rationalized by the charge donation of
one supplementary RS− ligand in 2 with a RS−–Cu ratio of 2,
while this ratio is 1 for the type 1 copper centres, CuA and 1.
We were able to obtain crystals of the CuI species in the pres-
ence of K+ in THF.‡ The X-ray structure corresponds to
an unprecedented hetero-tetranuclear Cu2/K2 complex,
[Cu2K2L2(THF)2] (3) (Fig. 4). A similar structure has also been
obtained with Na+ instead of K+ (data not shown). In 3, the two
K+ cations bridge two mononuclear [CuL]− units, avoiding a
direct Cu⋯Cu interaction (Cu⋯Cu = 5.301 Å). Indeed, each K+
atom bridges the two [CuL]− units asymmetrically via one thio-
late to one side and via two thiolates to the other. The remarkable
increase of the Cu–S–S/Cu–N–N angles in 3 (60.31 and 59.13°)
The redox potential of 2/2− (−1.3 V) is lower than that of 1+/1
(−0.79 V).7 Of the relevant biological transfer agents, the mono-
nuclear type 1 copper site of the plastocyanin (+0.37 V vs.
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terms, the outer-sphere (λo) and inner-sphere reorganization ener-
gies (λi). As λo is almost constant for a class of compounds in a
given solvent, we have calculated λi by DFT, which represents
the energy increment associated with the structural change of the
reactants in order to proceed from the initial to the final states.
The calculated λi values, 43.5 and 169.0 kJ mol−1 for 2/2− and
1+/1, respectively, are consistent with the fact that the mono-
nuclear redox 2/2− system should be faster than the dinuclear
1+/1 one. This result is opposite to that observed in biological
systems, for which it has been shown that the reorganization
energy is larger for the blue copper proteins than for the CuA
centre of the cytochrome c oxidase.26–29
The rigidity of the N2S2 ligand prevents important structural
modifications between the two oxidation states of the 2/2−
system, leading to a small reorganization energy which is of the
same order of magnitude as that found for the proteins.26–29 In
particular, the ligand preorganizes a six-membered copper-aza-
thiacyclohexane ring, which offers a geometry between a tetra-
hedral and a square planar, ideally designed for stabilizing both
CuII and CuI, thus explaining the low reorganization energy of
this model.
Fig. 3 Cyclic voltammogram of: (a) a saturated suspension of 2 in
DMF, 0.1 M Bu4NPF6 at a vitreous carbon electrode; (b) after an exhaus-
tive electrolysis at −1.30 V (formation of completely soluble 2−,
1 mM); v = 100 mV s−1. Potentials are given vs. Fc+/Fc.
On the other hand, the larger λi observed for the dinuclear
1+/1 system can be related to the notable increase of the Cu⋯Cu
distance during the oxidation of 1 leading to significant structural
modifications around the two Cu ions.7 Conversely, in CuA, the
Cu⋯Cu distance does not vary between the two formal oxi-
dation states, thus ensuring a low reorganization energy. Indeed,
in contrast to the model complexes, the copper sites of the bio-
logical electron transfer agents are confined inside the polypep-
tide chain thus avoiding significant structural modifications
between the two redox states. Therefore the nuclearity of the
system becomes a critical factor in ensuring the best possible
delocalization of the unpaired electron in the oxidized form,
explaining the better efficiency of CuA with respect to the type 1
copper centre.
Fig. 4 Molecular drawing of [Cu2K2L2(THF)2]·0.5THF (3·0.5THF).
The thermal ellipsoids are drawn at 30% probability level. All hydrogen
atoms and cocrystallized solvent molecules are omitted for clarity. Both
Cu ions display identical coordination environments and, as a result,
only one is highlighted. Selected bond distances (Å): Cu(1)–S(1) =
2.2940(14), Cu(1)–S(2) = 2.2512(16), Cu(1)–N(1) = 2.058(5), Cu(1)–N
(2) = 2.011(4).
Acknowledgements
MG thanks the CNRS for his post-doctoral fellowship.
Notes and references
‡Crystal data for 2: C38H30N2S2Cu, Mr = 642.34, triclinic, space group
with respect to 2 (35°) is consistent with different oxidation
states of the copper centres.
ˉ
P1, a = 9.109(2), b = 12.802(3), c = 13.188(2) Å, α = 88.37(1), β =
76.90(2), γ = 84.81(2), V = 1491.7(6) Å3, Z = 2, T = 200 K, Dc
=
=
1.430 g cm−3, 19 203 reflections collected with 5202 unique (Rint
The [CuIL]− unit is very reactive. It immediately reacts with
CH2Cl2 producing 1 via presumably an SN2 mechanism as was
proposed for comparable cases with Ni and Ru complexes.19,20
In the presence of K+, this CuI species leads to 3 in which the
[CuIL]− units are isolated from each other by K+ bridges. Note
that such a structure is very unusual since, to the best of our
knowledge, no mononuclear aliphatic thiolate CuI complex has
been described in the literature so far. Indeed, aromatic thiolates
are required for stabilizing the CuI state, but even with such
ligands mononuclear complexes are scarce.21–24
0.1532), R1 (wR2) = 0.0653 (0.0672) for 5202 reflections with I > 2σ(I).
CCDC 854562. Crystal data for 3: C86H80N4O2.5S4Cu2K2, Mr
=
1543.06, monoclinic, space group P21/c, a = 15.7411(4), b = 14.8294
(4), c = 32.7134(13) Å, α = 90, β = 91.330(3), γ = 90, V = 7634.2(4)
Å3, Z = 4, T = 150 K, Dc = 1.343 g cm−3, 29 519 reflections collected
with 15 396 unique (Rint = 0.0537), R1 (wR2) = 0.0635 (0.1499) for
15 396 reflections with I > 2σ(I). The 55 restraints correspond to the
geometry and displacement parameters of THF solvent molecule: Dis-
tances between adjacent atoms were restraint to 1.4 and 1.45 Å for O–C
and C–C distances respectively, and also 2.3 Å for O3–C112, C111–
C113, C112–C114, C113–O3 distances. Displacement parameters were
refined to be closer from Ueq (ISOR function), similar to the neighbour-
ing atoms (SIMU function) and in the direction of the bond (DELU
function). CCDC 854563.
In order to compare the capability of the two 1+/1 and 2/2−
redox systems to transfer electrons, we have calculated their re-
organization energy (λ), a parameter related to the electron trans-
fer kinetics. According to Marcus,25 λ can be separated in two
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